Aquaculture Oysters Succumb to Mystery Killer

Hannah Brown, a lab specialist working for the Virginia
 Institute of Marine Science, shovels oysters onto a table. 

In the Bay and elsewhere. Bay Journal, Study seeks to get to bottom of mysterious die-offs at oyster farms

For several years, an unexplained die-off of farm-raised oysters from the Chesapeake Bay to the Gulf of Mexico has perplexed industry members and the scientific community alike.

What they do know: The bivalves tend to die between May and early July. Most of the victims are market-size oysters or nearly there. And it mainly lays waste to triploid oysters, the type favored by farmers — and many diners, whether they realize it or not — for their faster growth and higher meat quality.

Researchers with the Virginia Institute of Marine Science are trying to uncover the cause before it seriously disrupts the state’s burgeoning $60 million oyster aquaculture sector. (Episodes have been reported in Maryland waters as well.) Responding to the industry’s growing calls for answers, the National Oceanic and Atmospheric Administration has awarded $300,000 to the institution to sift through clues turned up by fresh field surveys and lab experiments.

“There’s a lot about oysters we don’t know yet,” said Bruce Vogt, president of Big Island Aquaculture, an oyster farm based in a creek near the mouth of Virginia’s York River. He estimated that he loses 30–40% of his triploids annually to the mysterious die-off. “It’s critical we get an understanding of this.”

The phenomenon was first reported in 2012. Most aquaculture operations reported die-offs for around 30% of their oyster stock, but some in the lower end of the Chesapeake saw losses of 50–85%.

Such die-offs aren’t out of the ordinary for oysters, said Hamish Small, who is heading the VIMS study. The bottom dwellers are known to succumb in large numbers to insults such as diseases, toxic algae blooms, sudden drops in dissolved oxygen, and too much or not enough salt in the water.

“Much like us, they’re constantly dealing with stress of some sort,” Small said.

But the die-offs that began in 2012 were different, he explained. Growers couldn’t connect it to any of the typical causes. VIMS researchers were similarly confounded when they conducted their own analysis of oyster health at five sites in the Virginia portion of the Bay in 2014–15. Nothing fit.

Maybe triploid oysters just aren't the answer. 

The Wombat has Rule 5 Sunday: Salma Hayek up on time at The Other McCain.